Diesel Merc not Getting up to Speed

I’m driving my friend’s 70s Mercedes with a Diesel engine while he’s out of town. I know he’s been trying a few different things, but it’s still incredibly inconsistent. Some days (like this morning) it was fantastic - rode on the highway completely up to speed, made it up big hills, etc. Other days it won’t go over 35mph. He changed the fuel filter a couple weeks ago and that helped a bit, but it’s still pretty inconsistent. Any ideas?

Here’s what a 300D owner and car expert said when asked a very similar question:
"Hard to say without driving it but you might try some very strong injector cleaner and give it a good run. Also make sure all your turbo plumbing is very tight and secure. If none of that works, a valve adjustment and injection pump timing are probably a good next step. Many of these old diesels are severely neglected with regards to valve adjustment, dirty fuel, worn out injectors (they wear just like any other part) and weak glow plugs (also a replacement item that is overlooked). When I bought my pristine 300D, it took about 3 months and $1,500 to clear up all the mistakes, neglect and omissions. Hopefully you’ll have a smoother road. But if this is a high miler, certainly there are wear items. "

Sounds like a damn-near decent place to start! Thanks!